Will a common goldfish attack a fancy goldfish?
Well that's what I got told today at pets at home and I know that they don't because I had a fancy and common goldfish together. The person who told me this said there was all female guppies in a tank and I pointed out one was a male and he said it wasn't then he looked and it was a male. Lol educated by a kid

I don't think they will "attack" per se if the tank is big enough. But a normal, single-tailed fish-shaped goldfish has a big advantage in swimming speed and maneuverability compared to a double-tail and a bubble eye may have trouble with vision and moving. So the normal fish will get the largest amount of food and then grow faster and goldfish do get aggressive toward smaller fish in tanks smaller than ponds. So while I don't predict an immediate calamity, it is not a good long-term plan.